The invention generally relates to hearing aids and particularly relates to cochlear implants. Hearing impairment is believed to affect approximately 10% of individuals in the world. Many such individuals are in need of cochlear implants or hearing aids to listen to normal sounds. Conventional hearing instruments are designed to fit inside the ear, requiring small, low power, wide-dynamic-range front ends with a minimum of external components and good power supply rejection.
The use of wideband clock and telemetry signals in hearing instruments further requires a very high level of superb power-supply rejection in-band as well as at high frequencies. A fully analog hearing instrument should exhibit good power-supply rejection properties in all of its stages prior to rectification. A digital signal processor (DSP)-based hearing instrument should also have good power supply rejection in the analog front-end to ensure that the analog-to-digital (A/D) system is not exposed to aliasing and distortion errors caused by high-frequency supply noise.
Further, wide dynamic range is needed to meet patient needs in noisy environments. Hearing instruments are typically limited to 83 dB of input dynamic range by available microphone technology. Peak signals therefore, of 110 dBSPL and a microphone noise-floor of 27 dBSPL, make most hearing tasks possible. Typical low power microphones such as JFET buffered microphones with an electret capacitor provide an output voltage signal with good dynamic range, but that may fluctuate slightly with fluctuations in the power supply. Such fluctuations, or noise, may significantly detract from the microphone and pre-amplifier performance.
There is a need therefore, for improved low power wide-dynamic range microphone systems for hearing aids and cochlear implants. There is further a need for low power microphone systems that provide improved power supply noise rejection.